Track circuit



July 19, 1938. F X, REES 2,123,966

` TRACK CIRCUIT Original Filed Aug. 21, 1955 Patented July 19, 1938 UNITED STAFES PARENT QFFICE TRACK CIRCUIT Original application August 21, 1936, Serial No.

97,2011. Divided and this application Gctober 27, 1937, Serial No. 171,379

5 Ciaims.

This invention relates to improvements in the Well-known track circuits for railroads, and more particularly to rectified alternating current track circuits.

The primary object of the present invention is to improve the shunting operation of the usual tractive soft iron armature track relay of direct current track circuits under varying conditions of rail surface and ballast resistance.

This invention is in the nature of a variation or modification for carrying out the principles and mode of operation characteristic of the track circuit organization disclosed and claimed in my prior application Ser. No. 24,074, led May 29, 1935, and no claim is made herein to the invention disclosed in said prior application. The present application is also a division of my prior application Ser. No. 97,204 filed August 21, 1936 and no claim is made herein to the invention disclosed in the parent application as noW amended.

The reliable operation of the usual track circuit depends upon establishing a conducting path of low resistance through the axles and Wheels of a car or train; and the Weight of the equipment and condition of the rail surface are important factors affecting the resistance of a Wheel shunt. For example, Where the track rails of sidings, Crossovers, and the like are infrequently used, a coating of rust, usually an iron oxide, accumulates on the rail surface, and makes any Wheel shunt of high resistance for the relatively small inter-rail voltages commonly employed; and even Where train movement is frequent and the rail surface appears clean, a coating or film in the form of an oxide of silicon, or some other stable compound, is present on the rail surface, and offers a relatively high effective resistance for low voltages. It is found that these various coatings or films on the rail surface, apparently because they are in the nature of a stable chemical compound or composition, will break down and become a low resistance conducting path upon the application of sufficient voltage, Which may be termed an ionization voltage.

Generally speaking, and Without attempting to denne the exact nature and scope of the invention, it is proposed to provide a transformer of special construction, conveniently termed a peakervoltage transformer for supplying rectified current from an alternating current source to the ordinary direct current track circuit, so that relatively high peak voltages of short duration are obtained, as compared With a rectified Wave of the usual sine Wave shape of voltage required for the average or eifective relay energizing current, thereby serving to break down or ionize the film or coating on the rail surface and render more effective the shunting action of the Wheels and axles of a train or trains Without increasing the average current.

Various characteristic features, attributes, and advantages of the invention will be in part apparent, and in part pointed out, as the description progresses.

The accompanying drawing illustrates in a simplied and dragrammatic manner certain specific embodiments of the invention, the parts and circuits being shown more With the View of facilitating an explanation and understanding of the invention than for the purpose of showing in detail the structural organizations to be employed in practice.

In the drawing, Fig. 1 illustrates one form of the invention, and 1Eig. 2 is an explanatory diagram illustrating how the negative Waves of the current are blocked out by the rectifier shownin Fig. 1.

One important characteristic of this invention is that it assures reliable shunting of the ordinary track circuit under unfavorable conditions of rail surface, Weight of equipment, and ballast conditions. This desirable characteristic is due to the application of high peaked voltages to break down the lm or coating on the track rails, thereby assuring a shunting effect of the Wheels and axles of a car or train under conditions Where a lower voltage would be insuiiicient.

In accordance with the invention illustrate-d in Fig. l., it is proposed to employ a track relay of the usual direct current tractive armature type but having a substantial value of inductive reactance, and to supply the track circuit with time spaced unidirectional voltage pulses, instead of voltage pulses in opposite directions as shown in Fig. l. These unidirectional voltage pulses are preferably derived from alternating current but preferably have their peaks materially in excess of that of the usual sine Wave of alternating current, these peak voltages also being in excess of the steady operating voltage that Would be necessary for operating the track relay, thus tending to more easily overcome the resistance of the Wheel contact and establish a better ratio between maximum voltage and wheel shunt resistance. In this construction the ballast resistance between the track rails, together with an additional shunt or bleeder resistance as may be rcquired, constituting a conducting path into which the relay may discharge current during the time series.

intervals between the impressed voltage pulses; and the voltages and resistances being so proportioned that the rate of decay of current through the relay varies with the ballast resistance in such a way as to compensate for the change in the inter-rail impressed voltage with changes in ballast resistance, thereby maintaining a substantially constant average operating current through the track relay throughout a wide range of variations in ballast resistance.

Referring to Fig. 1 of the drawing, it is contemplated that the track rails H wil be joined together in the usual way, with insulated joints l2 designating the ends of the track sections, one of which only is illustrated. At one end of the track circuit, preferably at the entering end, the direction of traffic being in the direction of the arrow, the direct current track relay T4 of the usual tractive armature type construction is connected across the track rails Il, the resistance and/or inductance of this track relay T4 being dependent. upon the nature of the ballast resistance and a number of other factors, as will presently appear.

It is contemplated that the impedance of track relay T4 and its connections to the track rails will be some two or six times greater than ordinarily used, as for example, where ordinarily aV 4 ohm track relay7 would be used, a track relay of about 25 ohms would be employed. The purpose for'such relatively large impedance of the track relay and its connections is to permit the use of a greater inter-rail potential for overcoming the resistance of the wheel contact, and fur- Vther to supply sufficient energy to the track relay by smaller average current than would be necessary for the'usual 4 ohm relay.

In accordance with the invention the track relay is supplied with current of unidirectional time kspaced pulses, the direction of the pulses and space between them, as well as the peak voltage, being chosen with due regard to the other characteristics of the circuit., so as. to obtain the desired operating characteristics of the track circuit. In the embodiment shown in Fig. 1, a source of alternating current of an ordinary frequency, such as 60 cycles, is employed. Such source of current is shown conventionally by the generator G4. This source of alternating current G4 is supplied directly to the primary winding 'i4 of a peaking transformer PT4. The secondary winding 84 of this peaking transformer PT4 is connected to the track rails through an adjustable resistance IQ and a rectifier l5 connected in This adjustable resistance I9 constitutes the usual limiting resistance. In certain applications of the invention, for reasons hereinafter explained, it is also desirable to employ a bleeder or regulating shunt resistance 2|, preferably adjustable, which is connected across the track rails, as shown, preferably at the feed end of the track circuit'.

In the arrangements shown in Fig. 1, the rectier i5 serves to impress across the track rails waves or half-cycles of peaked alternating current Voltage supplied by the secondary winding 84 of the peaking transformer PT4, thereby providing a single unidirectional impulse for each cycle of alternating current as shown by the Waves C5 of Fig 2. It may be pointed out that the curves shown in Fig. 2 are substantiallx7 the same as those shown in Fig, 3 of the parent application except that the negative voltage waves shown in Fig. 3 of the parent application have been blocked out "by the rectifier l5 and are therefore not shown in Fig. 2. Certain types of rectifiers, such as the Wel1-known copper-oxide dry plate type, have the property or characteristic that the resistance to the ow of current in the direction in which the rectifier is conducting, varies invariably with the current induced, such forward resistance of the rectifier increasing as the conducted current decreases. This characteristic or property of the rectier I5 is a factor in connection with other features of this form of the invention to compensate for variations in ballast resistance.

Considering now the operation of the track circuit organization of the construction shown in Fig. l, and the theory believed to underlie the results obtained, the impressed Voltage applied to the track rails at the feed end comprises unidirectional time spaced pulses or waves. These voltage pulses supply current to the ballast resistance and to the track relay through the limit ing resistance I9; and each wave causes a corresponding temporary increase in the current owing in the track relay. As an impressed voltage wave is applied to the relay current through this relay increases, with a time lag due to the inductive reactance of the relay; and as this impressed voltage wave falls, the current through the relay decreases. As thc relay current decreases, the collapse or decay of flux in the relay induces a voltage in its winding in a direction to sustain current flow in the relay, and in accordance With established principles, the rate of decrease or decay of current is dependent upon the inductance of the relay and the resistance of the multiple path which in the present instance is the ballast resistance of the track circuit. It is found that the rate at which the relay current will flow between two successive waves of unidirectional potential, depends upon the. joint effect of the ballast resistance and the bleeder resistance 2l (which bleeder resistance in track circuits may be omitted). It is thus seen that the decay of current owing in the relay T1 during the time interval between voltage pulses is at a slower rate with a lower ballast resistance, and that the greater rate of decay of current will take place for higher ballast resistance.

The pick-up and drop-away current value of the track relay T4 Adepends upon what may be termed the average current through it; and it can be seen that the magnitude of this average current depends upon the rate at which the relay current decreases or decays during the half-cycles where the impressed voltage is cut o, as well as upon the value of the voltage pulses. In other Words, the higher the pulses of impressed voltage, the greater the instantaneous current; and also the slower the rate of decay of relay current, the greater will be the average current through the relay; and vice versa.

In the ordinary track circuit, when the battery voltage and limiting resistance has been selected or adjusted to provide sufficient current for the track relay under wet ballast conditions, that is, low ballast resistance, if the ballast dries out and its resistance increases, less current flows through the ballast and the limiting resistance, the voltage drop through the limiting resistance then becomes smaller, and a greater voltage is impressed across the track rails to increase the current through the relay. It can thus be seen that, if once the ordinary track circuit has been adjusted to hold up the relay under wet ballast conditions, there will be excessive current through the relay under dry ballast conditions, which materially interferes with the shunting of the track relay. For this reason, it is desirable to provide something Which will act automatically to maintain a uniform normal current through the track relay under varying ballast conditions, so that the energization of the relay is substantially the same at all times, and so that the relay can be shunted quickly and reliably under both wet and dry ballast conditions. An important feature of the invention shown in Fig. l is the automatic cornpensation provided to maintain the relay current substantially uniform for various ballast resistances throughout a wide range at the higher ballast resistances and above what may be termed the zone of Wheel shunt resistance.

Explaining the theory apparently underlying the operation accomplishing this result, it appears that changes in the voltage applied to the relay with variation in ballast resistance are accomplished by compensating change in the rate of decay of the relay current between the voltage pulses, in such a way that, with suitable proportioned parts, the average operating current through the relay is maintained substantially uniform throughout a Wide range of ballast resistances above the lower values, around 2 ohms per thousand feet of track.

In practicing this invention, the parts are selected and proportioned to provide adequate operating current for the track relay T"z under the most unfavorable or Wet ballast conditions to be encountered; but as the ballast dries out, less current is supplied to the track circuit as a whole, and the voltage drop through the limiting resistance is decreases to raise the voltage impressed upon the relay T4, such increase in ballast resistance is accomplished by a more rapid decay of relay current between the pulses of impressed voltage, so that the average current iiowing in the track relay T4 remains substantially constant, rather than to increase steadily with. an increase in 1ballast resistance as is the case in the ordinary track circuit. Consequently, the track relay T4 is not overenergized under dry ballast conditions and may be shunted as surely as quickly under dry as under wet ballast conditions.

In some applications of the invention, it is found that there is what may be termed overcompensation, such that the average current through the relay of an unoccupied track circuit decreases at the higher ballast resistances to the point where it will fail to hold up its armature. This appears to be due to a relatively greater change in relay current between the voltage pulses for higher ballast resistances, than changes in the impressed voltage, with the result that the average current actually decreases for higher values of ballast resistances. In such cases, the shunt or bleeder resistance 2l across the track relay is employed to provide what may be considered to be an artiiicial ballast resistance or leakage path for the purpose of maintaining a su'iciently low resistance for the discharge circuit for the track relay T4 under very dry ballast conditions to avoid too quick a decay in the relay current between the voltage pulses and thereby maintain the average current. This shunt or bleeder resistance 2i is preferably adjustable, and with the proper selection or adjustment thereof, together with the adjustment aiiorded by the usual limiting resistance i9, it appears that the desired regulation of relay current may be obtained for track circuits for various lengths and ballast materials throughout the full operating range encountered in practice.

The maintenance of substantially constant average current through the relay for different ballast resistances seems to be helped also by the current-resistance characteristics of the copperoxide rectifier l5 in the arrangement shown in Fig. l, the eifect of the rectifier in this respect, however, being apparently dependent upon a number of related factors and dicult to explain and evaluate. A source of alternating current derived from a peaker transformer when used in combination with a half-wave rectifier l5, affords a convenient way of obtaining the time spaced impulses of impressed Voltage, and with such an arrangement the operating characteristics of the rectifier may also be taken advantage of to modify the current regulating characteristics of the track circuit. For example, assuming a setting for Wet ballast conditions, when the ballast dries out and less current is supplied to the track, the forward resistance of the rectier, i. e. the resistance in the direction in which it is conducting, increases and in effect adds to the limiting resistance i9 to maintain a voltage drop wth a smaller current to give nearly the same impressed voltage across the track rails. For this reason, and in this way, the characteristics of the rectifier are helpful in obtaining the desired current regulation.

Another important feature of this invention is that the peak voltage of the half-cycles of peaked alternating current are much in excess of the steady direct current voltage that would be required to produce the same general operating current through the relay. In other words, the peak voltages applied across the track rails in accordance with the form of invention shown in Fig. 1 are much greater than the steady voltage that would be used with the same type of track relay in a continuous direct current track circuit. Such peak voltages very materially assist in reducing or breaking down the resistance of the wheel contacts and establish an effective low resistance wheel shunt, this being true more particularly with dirty or rusty track rails, or with lightweight equipment, such as gas-electric cars.

'Ihe resistance of the Wheel Contact is found to vary greatly with the conditions of the rail surface and the weight of the equipment; and it appears that in many cases voltages much higher than the relatively small voltages, of about 2 volts normally employed for track circuits, are necessary to break down the resistance of the Wheel contact to a point where ionization and actual shunting of the track relay takes place. 'Ihe peak voltages provided by this invention, which are many times those commonly used in track circuits, apparently break down the resistance at the wheel contact by an ionization eifect, and enable effective shunting with dirty or rusty track rails and light-weight equipment in a manner not obtainable with the ordinary track circuit arrangement.

In this connection, it will be noted that the peak voltages are periodically applied at frequent intervals and are available to break down the resistance of the Wheel contact, as the car or train moves along the track and its Wheels contact with successive points on the rails, and with the resistance once broken down it will remain of low value until the train has left and an opportunity for soiling and oxidization of the rail surface has been presented.

Further, the use of a pulsating current obtaining from a peaked alternating current source through the medium of a half-wave rectifier,

as distinctive from a steady current for energizing the track relay, acts to improve the pick-up and drop-away characteristics of the high impedance soft iron armature track relay T4. In other Words, a direct current tractive armature type relay of the usual construction will pick up on less current and drop away on a greater current When energized with half-cycles of rectified alternating current or equivalent time spaced current impulses, than When energized With an ordinary unidirectional voltage from a battery or the like, and this effect is still more pronounced When these half-cycles are of extremely peaked Wave form. This is attributed to the fluctuating nature of the energizing current which tends to initiate armature movement at critical current values in a manner that does not occur with steady energization. Also, the movement of the armature of the relay is accompanied by a flux change Which appears to be a contributing factor to the superior ratio of drop-away current to pick-up current obtained by the use of pulsating energization of the track relay in accordance with this invention.

From the foregoing it will be appreciated that this invention involves certain selection and proportioning of parts in the complete organization; and the various voltages, resistances, and inductive reactances are so completely interrelated that a variation in the change in one factor tends to modify one or more of the other factors.

Generally speaking, the underlying principles and advantages of the invention shown in Fig. l may be obtained when the limiting resistance I9 and the shunt or bleeder resistance 2l, if used, are selected to conform With the length of the track circuit, condition of the ballast, range of variation in ballast resistance, and the like; the inductive reactance of the relay T4 is preferably so chosen as to require peak voltages suitable for reducing the resistance of the wheel contact by breaking doWn the resistance by ionization; and these various factors are relatively proportioned to provide the necessary average current through the track relay of an unoccupied track circuit under Wet ballast conditions, and to maintain such average current substantially constant throughout the range of variation of ballast resistance encountered with changing weather conditions.

The particular embodiments of the invention shown and described are merely illustrative; and various adaptations, modifications, and additions may be employed, Without departing from the principles and mode of operation of the invention.

What I claim isz- 1. A track circuit for railroads comprising a half-Wave rectifier, a limiting resistance, a peaking transformer energized from a source of alternating current for impressing voltage across the track rails at one end of the track section in series with said rectifier and said limiting resistance, and a shunt bleeder resistance connected across the track rails to cooperate with the ballast resistance to provide a conducting path through which the track relay having considerable inductive reactance may discharge current during the half cycles when the impressed `voltage is blocked by the rectifier.

2. A track circuit for railroads having the usual variation in ballast resistance between the track rails and comprising, a direct current track relay of the tractive armature type connected yacross the track rails at one end of the track section, a peaking transformer having an in-put and an out-put Winding, a source of alternating current connected to the in-put Winding of said peaking transformer, means including a half- Wave rectifier and a limiting resistance in series for connecting the out-put Winding of said peaking transformer to the track rails at the other end of said track section to impress peak voltages substantially greater than the steady operating voltage for said relay, a shunt bleeder resistance connected across the track rails, the magnitude of said peak voltages and their time spacing being selected With respect to the selfinduction of said track relay and the resistance of said bleeder to provide an approximately uniform operating current for said track relay throughout a Wide variation in ballast resistance.

3. A track circuit for railroads comprising, a direct current track relay of the usual tractive armature type connected across the track rails at one end of the track section, a source of alternating current, a transformer having a high leakage reactance, an adjustable limiting resistance, a half-Wave rectifier, and means connecting said source of alternating current to the in-put side of said transformer and connecting the out-put side of said transformer across the track rails at the other end of said track section through said limiting resistance and said half-Wave rectifier, a bleeder resistance connected across the track rails of said section, the self-induction of said track relay and the resistance of said bleeder being such as to tend to maintain current of a value in its Winding to keep its armature attracted between the half cycles of impressed voltage, and the peak voltages of the half cycles of impressed voltage being such as to provide an average current through said track relay comparable with its steady operating current and being materially greater than the voltage for such steady operating current, whereby peak voltages across the track rails suitable for breaking down anycontact resistance of the Wheel shunt are available Without a correspondingly greater average energization of the track relay which Would interfere with its shunting.

Ll. A track circuit for railroads comprising, a direct current track relay of the usual tractive armature type having its winding connected across the track rails at one end of a track section, a limiting resistance, and means including a transformer having high leakage reactance and a rectifier included in its secondary Winding for supplying time spaced uni-directional voltage pulses through said limiting resistance across the track rails at the other end of said section, a bleeder resistance connected across the track rails of said track section, the self-induction of said relay maintaining current through its -winding between such Voltage pulses by discharging into the ballast resistance and bleeder resistance, the rate of decay of said current of self-induction between the voltage pulses varying with changes in ballast resistance to compensate for the variations in current supplied to said track relay by the voltage pulses for different ballast resistance so as to cause approximately the same average current through the track relay for a Wide variation in ballast resistance.

5. A track circuit for railroads having the usual variation in ballast resistance and comprising, a direct current track relay of the usual tractive armature type having its Winding connected across the track rails at one end of a track section, an adjustable limiting resistance, a source of alternating current, a transformer having a. high leakage reactance and a rectifier included in series in its secondary Winding for impressing time spaced uni-directional Voltage pulses through said limiting resistance across the track rails at the other end of said track section, a bleeder resistance connected across the track rails of said track section, the time spacing and magnitude of said voltage pulses, the self-induction of said track relay, and said limiting resistance and bleeder resistance having such relation that approximately the same average current flows through said track relay for a Wide range of variation in ballast resistance.

FRANK X. BEES. 

